Method for producing capacitor, capacitor, wiring board, electronic device, and ic card

ABSTRACT

There is provided a method for producing a capacitor which is capable of producing a capacitor having a high withstand voltage and low leakage current,
         the method for producing a capacitor which is a method for producing a capacitor having a substrate serving as one electrode, a dielectric layer formed on top of the substrate, and the other electrode formed on top of the dielectric layer,   the method including a step for forming an amorphous titanium oxide layer which is to become the dielectric layer on top of the substrate by anodizing the substrate, which is composed of titanium or titanium alloy, in an electrolyte solution containing hydrogen peroxide and having a temperature of 3° C. or less; and a step for forming the other electrode on top of the dielectric layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 12/762,071 filed Apr. 16, 2010, which is a Continuation ofInternational Application No. PCT/JP2008/068647 filed Oct. 15, 2008,which claims benefit to Japanese Patent Application No. 2007-270433filed Oct. 17, 2007. The above-noted applications are incorporatedherein by reference to their entirety.

TECHNICAL FIELD

The present invention relates to a method for producing a capacitor, acapacitor, and a wiring board, electronic device and IC card whichinclude the capacitor, and particularly relates to a method forproducing a capacitor which is capable of producing a capacitor having ahigh withstand voltage and low leakage current.

BACKGROUND ART

Aluminum oxide, tantalum oxide and niobium oxide have conventionallybeen used as dielectric materials of electrolytic capacitors. Inaddition, studies have long been conducted on capacitors using titaniumdioxide, which has a larger relative dielectric constant than the aboveoxides, for the dielectric material (hereafter, frequently referred toas a “titanium capacitor”). However, due to the problem of large leakagecurrent, these titanium capacitors have yet to be put to practical use.This problem of large leakage current is critical, especially for themetallic electrodes effective in reducing the extent of impedance in thegigahertz region which has been required recently, since restoration(that is, reoxidation) of electrical leakage, which takes place when anelectrolyte solution or electrically conductive polymer is used for thecathode in an electrolytic capacitor, cannot be expected.

The following describes previous attempts made to reduce the level ofleakage current in titanium capacitors.

For example, in Patent Document 1, although a non-aqueous solvent issubjected to anodic oxidation for use as an electrolyte solution, thereis a description stating that “a product chemically converted in anon-aqueous solution instantly deteriorates when transferred to anaqueous solution”. Consequently, although not described in PatentDocument 1, it is clear that a non-aqueous solvent having low electricalconductivity is used as an electrolyte solution. In addition, possiblydue to the use of a non-aqueous solvent as an electrolyte solution,although the leakage current is low, dielectric loss tangent at 10 kHzis 10% or more in all cases.

Further, in Patent Document 2, a method has been shown for obtaining ananodized film having superior electrical properties by using a titaniumalloy containing vanadium, chromium and aluminum. However, thedielectric loss tangent thereof is 1.5% or more.

Moreover, in Patent Document 3, there is a description stating that acapacitor obtained by anodic oxidation of titanium has leakage currentthat is greater than that of tantalum or aluminum by two digits or more.Furthermore, in Patent Document 3, a method has been shown for reducingleakage current by forming a passive layer with a nitric acid solutionas a pretreatment of anodic oxidation. However, the dielectric losstangent of the resulting sample is 1.5% or more.

Also, in Patent Document 4, the same inventor as that of Patent Document3 has shown that the addition of tungsten or molybdenum to titaniumreduces the level of leakage current to about one-half, as compared tothe cases where no addition was made. However, even though the leakagecurrent problem is improved by reducing the level thereof down to adegree of ½, this level is still inadequate for practical use.

In addition, Patent Document 5 has shown that leakage current and losscan be reduced by containing barium peroxide or strontium peroxide in amolten salt of sodium nitrite and anodizing at a temperature of 280 to350° C. However, the dielectric loss tangent at this time is 2.8% ormore.

Further, Patent Document 6 has shown that leakage current is reduced byusing an alloy containing 20 to 30 atomic % of aluminum in titanium.However, in Patent Document 6, measurement of electrical properties hasbeen carried out in an electrolyte solution. It has generally been knownthat in electrostatic capacitors, an electrical leakage portion isreanodized and insulated (restoration effects) when a direct currentvoltage is applied in an electrolyte solution, electrically conductivepolymer and the like. Therefore, it is assumed that the level of leakagecurrent was reduced due to the restoration effects in the measurementmade in Patent Document 6.

In addition, in Patent Document 7, a method has been shown for obtaininga capacitor having a satisfactory dielectric loss tangent by adjustingthe anodic oxidation conditions and carrying out a heat treatmentthereafter. In Patent Document 7, there is a description stating thatlow temperatures are more desirable for anodic oxidation, and a casewhere an anodic oxidation process was carried out at a temperature of 5°C. is disclosed therein as an example. However, although the electricalproperties of capacitors were measured in Patent Document 7 by employingan electrolyte solution having a restoration capacity as a cathode forthe capacitors, the dielectric loss tangent thereof exceeded 0.6%.

Further, in Non-Patent Document 1, it has been shown that the dielectricconstant of an anodized film of titanium is dependent on the temperaturefor anodic oxidation. However, the dielectric constant reduced as thetemperature decreased, and the relative dielectric constant at 303 K(that is, 30° C.) was 26.2. It is difficult to produce a capacitorhaving a large capacity with this dielectric constant.

-   [Patent Document 1] Japanese Examined Patent Application, Second    Publication No. Sho 33-5816-   [Patent Document 2] U.S. Pat. No. 3,126,503-   [Patent Document 3] Japanese Examined Patent Application, Second    Publication No. Sho 42-27011-   [Patent Document 4] Japanese Examined Patent Application, Second    Publication No. Sho 42-24103-   [Patent Document 5] Japanese Examined Patent Application, Second    Publication No. Sho 43-2649-   [Patent Document 6] Japanese Examined Patent Application, Second    Publication No. Sho 54-1020-   [Patent Document 7] Japanese Unexamined Patent Application, First    Publication No. Hei 5-121275-   [Non-Patent Document 1] Corrosion Science, Vol. 37, No. 1, pp.    133-144

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

In order to solve the above-mentioned problems regarding the titaniumcapacitors, an object of the present invention is to provide a methodfor producing a capacitor which is capable of producing a capacitorhaving a high withstand voltage and low leakage current.

Moreover, another object of the present invention is to provide acapacitor having a high withstand voltage and low leakage current.Furthermore, yet another object of the present invention is to provide awiring board, electronic device and IC card which include the capacitorof the present invention.

Means for Solving the Problems

In order to solve the above-mentioned problems, the present inventionprovide the following aspects.

(1) A method for producing a capacitor having a substrate serving as oneelectrode, a dielectric layer formed on top of the substrate, and theother electrode formed on top of the dielectric layer, the methodincluding a step for forming an amorphous titanium oxide layer which isto become the dielectric layer on top of the substrate by anodizing thesubstrate, which is composed of titanium or titanium alloy, in anelectrolyte solution containing hydrogen peroxide and having atemperature of 3° C. or less; and a step for forming the other electrodeon top of the dielectric layer.

(2) The method for producing a capacitor according to the above aspect(1), wherein the concentration of the hydrogen peroxide in theelectrolyte solution is from at least 0.1% by mass up to less than 50%by mass.

(3) The method for producing a capacitor according to the above aspect(1) or (2), wherein the electrolyte solution is an aqueous phosphoricacid solution.

(4) The method for producing a capacitor according to any one of theabove aspects (1) to (3), wherein the titanium alloy is an alloycontaining 70% by mass or more of titanium.

(5) The method for producing a capacitor according to any one of theabove aspects (1) to (4), wherein the substrate is a foil.

(6) The method for producing a capacitor according to the above aspect(5), wherein the thickness of the foil is within a range from 5 to 300μm.

(7) The method for producing a capacitor according to any one of theabove aspects (1) to (6), wherein the other electrode is composed of ametal.

(8) The method for producing a capacitor according to any one of theabove aspects (1) to (7) further including a step for laminating aninsulating material layer which is to become a dielectric layer on topof the titanium oxide layer.

(9) A capacitor produced using the method for producing a capacitordescribed in any one of the above aspects (1) to (8).

(10) A capacitor having a substrate serving as one electrode, adielectric layer formed on top of the substrate, and the other electrodeformed on top of the dielectric layer, the capacitor including thesubstrate composed of titanium or titanium alloy; and the dielectriclayer containing an amorphous titanium oxide layer.

(11) The capacitor according to the above aspect (10), wherein thedielectric layer is a laminated body including the amorphous titaniumoxide layer and an insulating material layer.

(12) The capacitor according to any one of the above aspects (9) to(11), wherein the refractive index of the amorphous titanium oxide layerat a wavelength of 632.8 nm is within a range from 1.90 to 2.35.

(13) The capacitor according to any one of the above aspects (9) to(12), wherein the relative dielectric constant of the amorphous titaniumoxide layer is within a range from 30 to 50.

(14) The capacitor according to any one of the above aspects (9) to(13), wherein the product of capacitance density and dielectricbreakdown voltage at a measuring frequency of 1 kHz is 200 nF·V/cm² ormore.

(15) The capacitor according to any one of the above aspects (9) to(14), wherein the dielectric loss tangent at a measuring frequency of 1kHz is 0.01 or less.

(16) The capacitor according to any one of the above aspects (9) to(15), wherein the electrostatic capacitance at a measuring frequency of1 MHz is 80% or more of the electrostatic capacitance at a measuringfrequency of 100 Hz.

(17) A wiring board including the capacitor described in any one of theabove aspects (9) to (15).

(18) An electronic device including the capacitor described in any oneof the above aspects (9) to (15).

(19) An IC card including the capacitor described in any one of theabove aspects (9) to (15).

Effect of the Invention

Since the method for producing a capacitor according to the presentinvention includes a step for forming an amorphous titanium oxide layer,which is to become a dielectric layer, on top of a substrate composed oftitanium or an alloy containing titanium by anodizing the substrate inan electrolyte solution containing hydrogen peroxide and having atemperature of 3° C. or less, a capacitor having a high withstandvoltage and low leakage current can readily be obtained.

In addition, according to the method for producing a capacitor of thepresent invention, the thickness of the amorphous titanium oxide layer,which is to become a dielectric layer, can readily be controlled byadjusting the conditions for subjecting the substrate to anodicoxidation.

Accordingly, according to the method for producing a capacitor of thepresent invention, a capacitor having a high withstand voltage and lowleakage current and including a dielectric layer with a desiredthickness can be produced at low cost without requiring the use ofcomplex and elaborate equipment.

Furthermore, according to the method for producing a capacitor of thepresent invention, since a capacitor having a low leakage current can beobtained, there is no need to use an electrolyte solution, electricallyconductive polymer or carbon paste and the like which is capable ofself-restoration as an electrode, and thus the size of the capacitor canbe reduced and the manufacturing process can also be simplified, ascompared to the cases where a capacitor having an electrode capable ofself-restoration is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view showing an example of acapacitor according to the present invention.

FIG. 2 is a schematic cross sectional view showing another example of acapacitor according to the present invention.

FIG. 3 is a schematic cross sectional view showing yet another exampleof a capacitor according to the present invention.

FIG. 4 is an SEM micrograph of the surface of a titanium oxide layer.

FIG. 5 is a TEM micrograph of a cross section of a titanium oxide layer.

FIG. 6 is a graph showing the result of energy dispersive X-rayspectroscopy (EDS) of a titanium oxide layer.

DESCRIPTION OF THE REFERENCE SYMBOLS

-   -   1: Substrate    -   2: Dielectric layer    -   3: Counter electrode (the other electrode)    -   10: Capacitor

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described below in great detail.

(Capacitor)

FIG. 1 is a schematic cross sectional view showing an example of acapacitor according to the present invention. A capacitor 10 shown inFIG. 1 includes a substrate 1 which also serves as one electrode, adielectric layer 2 formed on top of the substrate 1, and a counterelectrode (the other electrode) 3 formed on top of the dielectric layer2. The capacitor 10 of the present invention shown in FIG. 1 is acapacitor produced using a method for producing a capacitor according tothe present invention described later.

It is desirable that the thickness of the capacitor 10 be as thin aspossible with respect to compact size and increased functionalsophistication of the electronic device using the capacitor, and ispreferably set to a thickness of 200 μm or less. Further, it is moredesirable that the thickness of the capacitor 10 be 100 μm or less sinceit facilitates formation of a capacitor within a wiring board.

(Substrate)

The substrate 1 is composed of titanium or an alloy containing titanium.The titanium alloy may be any alloy as long as the dielectric layer 2composed of an amorphous titanium oxide layer can be formed on thesurface thereof by an anodic oxidation process described later, andalloys containing 70% by mass or more of titanium are preferably used,and specific examples thereof include β titanium (containing 76% by massof titanium, 15% by mass of V, 3% by mass of Cr, 3% by mass of Sn and 3%by mass of Al).

The shape of the substrate 1 may be any shape as long as it can be usedas an electrode of the capacitor 10, and, for example, a plate-shapedsubstrate, a foil-shaped substrate or the like can be used, although afoil-shaped substrate is particularly preferable.

When the substrate 1 is foil, the capacitor 10 can be easily reduced inboth size and weight. In addition, as the surface area of the substrate1 per unit mass increases, proportion of the substrate 1 with respect tothe dielectric layer 2 increases, and thus it is advantageous to attaina capacitor having a high capacity when the substrate 1 is foil.

When the substrate is foil, the thickness of the foil is preferablywithin a range from 5 to 300 μm, more preferably from 5 to 100 μm an andeven more preferably 5 to 30 μm. When the foil thickness exceeds theabove-mentioned range, the level of the electrostatic capacitance perunit volume of the capacitor 10 reduces. On the other hand, when thefoil thickness is less than the above-mentioned range, the foil becomestoo thin when an etching is carried out on the substrate 1, which makesit difficult to handle.

Further, as the substrate 1, a substrate having a roughened surface, asubstrate having fine pores on the surface and inside thereof, or thelike may be used. In this case, it is preferable because the surfacearea of the substrate 1 per unit mass becomes large, and thus it isadvantageous to attain a capacitor having a high capacity.

(Dielectric Layer)

The dielectric layer 2 is composed of an amorphous titanium oxide layer.When the cross section of titanium oxide layer constituting thedielectric layer 2 is observed using a transmission electron microscope(TEM), the layer appears amorphous with hardly any crystallized domainsvisible within the field of view. In addition, the refractive index ofthe titanium oxide layer constituting the dielectric layer 2 is within arange from 1.90 to 2.35 at a wavelength of 632.8 nm, a value smallerthan 2.56 which is the refractive index of a crystalline titaniumdioxide. From these observations, it is apparent that the titanium oxideconstituting the dielectric layer 2 is not crystalline but amorphous.Further, the relative dielectric constant of such a titanium oxide layerconstituting the dielectric layer 2 is typically within a range from 30to 50.

It is preferable that the thickness of the dielectric layer 2 be withina range from 1 nm to 300 nm. In terms of the thickness of the dielectriclayer 2, although the dielectric breakdown voltage reduces when it isthin, the capacitance of a capacitor increases. For this reason, thethickness of the dielectric layer 2 is appropriately determineddepending on the performance required for the capacitor 10, such as thelevel of withstand voltage and capacitance density of the capacitor 10.

(Counter Electrode)

The counter electrode 3 is preferably composed of a metal formeddirectly on top of the dielectric layer 2.

By forming the counter electrode 3 composed of a metal directly on topof the dielectric layer 2, it is possible to provide the capacitor 10with excellent high-frequency properties. Examples of the metal used inthe counter electrode 3 include copper, nickel, platinum, palladium andaluminum. Of these metals, it is most preferable to use copper which iseasy to handle during a soldering process or the like. The thickness ofthe counter electrode 3 can be determined depending on the material ofthe counter electrode 3 with no particular limitations, and thethickness can be adjusted to, for example, about 1 to 40 μm.

It should be noted that since the capacitor 10 of the present embodimenthas a low leakage current, it is not necessarily essential to use, asthe counter electrode, an electrolyte solution, electrically conductivepolymer or the like which is capable of self-restoring the dielectriclayer 2.

(Capacitor Characteristics)

In the capacitor 10 of the present embodiment shown in FIG. 1, theproduct of capacitance density and dielectric breakdown voltage at ameasuring frequency of 1 kHz is normally 200 nF×V/cm² or more (forexample, the capacitance density is 100 nF×V/cm² or more if thedielectric breakdown voltage is 2 V when applying a direct currentvoltage). Further, in the capacitor 10 of the present embodiment, thedielectric loss tangent (tan δ) at a measuring frequency of 1 kHz isnormally 0.01 or less, and the electrostatic capacitance at a measuringfrequency of 1 MHz is 80% or more of the electrostatic capacitance at ameasuring frequency of 100 Hz.

(Method for Producing a Capacitor)

Next, a detailed explanation is provided of the method for producing thecapacitor of the present embodiment using an example.

In order to produce the capacitor 10 of the present embodiment shown inFIG. 1, it is preferable to first remove natural oxide films, stains,scratches or the like from the surface of the substrate 1, byconducting, for example a pretreatment for removing the surface layer ofthe substrate 1 composed of titanium or a titanium alloy by at least 1μm through an etching process. As an etching method to be adopted in thepretreatment step, chemical etching using hydrofluoric acid orelectrolytic etching and the like can be employed. In addition, in orderto obtain the capacitor 10 with a high capacitance, by appropriatelyselecting the etching conditions in the pretreatment step, the surfacearea of the substrate 1 may be increased by forming irregularities onthe surface thereof.

Next, the substrate 1 (which may be cut out so as to have an adequatesize) with which the aforementioned pretreatment step has already beencompleted is anodized, thereby forming the dielectric layer 2 composedof an amorphous titanium oxide layer on the substrate 1.

When conducting an anodic oxidation on the substrate 1, it is preferableto carry out the process by first coating the substrate 1 with a maskingmaterial at a location in which the substrate 1 is brought into contactwith an electrolyte solution in order to avoid the adverse effects dueto the fluctuations in the liquid level when immersing the substrate 1in the electrolyte solution. Examples of the masking material include acommonly used heat resistant resin, preferably a heat resistant resinthat is either soluble or swellable in a solvent or a derivativethereof, and a composition composed of an inorganic fine powder andcellulose-based resin (for example, refer to Japanese Unexamined PatentApplication, First Publication No. Hei 11-80596).

Subsequently, a masking tape is attached onto one surface of thesubstrate 1 so as not to be subjected to anodic oxidation. Then ananodic oxidation process is carried out by conducting chemicalconversion at a predetermined voltage and current density using thesubstrate 1 as an anode by immersing the substrate 1 in the electrolytesolution while making the other surface thereof, to which the maskingtape is not attached, to face the cathode.

(Electrolyte Solution)

The electrolyte solution used for the anodic oxidation of the substrate1 is an electrolyte solution containing hydrogen peroxide and anelectrolyte. The concentration of hydrogen peroxide is preferablymaintained from at least 0.1% by mass up to less than 50% by mass, morepreferably from 0.1% by mass to 40% by mass, and even more preferablyfrom 0.2% by mass to 20% by mass. When the concentration of hydrogenperoxide either exceeds the above-mentioned range or is less than theabove-mentioned range, the dielectric loss tangent (tan δ) of 0.01 orless at a measuring frequency of 1 kHz cannot be achieved at times.

Although the detailed action mechanism of hydrogen peroxide is notclear, it is thought that one or more of the following effects areattained: i.e., an effect to prevent the reprecipitation of titaniumcompound on the anode due to the formation of peroxo complex withtitanium ions; an effect as a depolarizer to prevent the hydrogen gaswhich is said to be partially generated also on the anode from formingair bubbles and adversely affecting the anodic oxidation of thesubstrate 1; and an effect as an oxidizing agent to assist the anodicoxidation of the substrate 1.

Examples of the electrolyte contained in the electrolyte solutioninclude an acid and/or a salt thereof. More specific examples of theelectrolyte include phosphoric acid, sulfuric acid, oxalic acid, boricacid and adipic acid as well as salts thereof. It is particularlypreferable when an aqueous phosphoric acid solution containing, as anelectrolyte, at least one material selected from phosphoric acid and asalt thereof is used as an electrolyte solution, since the level of theresistance of titanium oxide layer obtained by the anodic oxidation isfurther increased. It is assumed that the reason for the aboveobservation is due to the prevention of crystallization of an anodizedfilm by the phosphorus incorporated within the anodized film during theanodic oxidation of the substrate 1. In addition, it is preferable touse an inorganic compound, such as phosphoric acid, which is hardlyoxidized as an electrolyte because the concentration of hydrogenperoxide within the electrolyte solution reduces only gradually.

Further, an antifreezing agent may be contained in the electrolytesolution. The amount of antifreezing agent added is preferably as smallas possible so that the electrolyte solution is not frozen during theanodic oxidation. Examples of the antifreezing agent include ethyleneglycol, isopropanol, ethanol and diethylene glycol.

(Electrolyte Solution Temperature)

The temperature of the electrolyte solution during anodic oxidation isadjusted to 3° C. or less, and is preferably adjusted to 0° C. or less.When the temperature of the electrolyte solution exceeds 3° C., thecrystallization of the titanium oxide layer readily occurs. Morespecifically, when the temperature of the electrolyte solution exceeds3° C., the refractive index of the titanium oxide layer obtained byanodic oxidation may exceed 2.35. By adjusting the temperature of theelectrolyte solution to 3° C. or less, an amorphous titanium oxide layercan be stably formed. In addition, when the temperature of theelectrolyte solution exceeds 0° C., air bubbles begin to form on thesubstrate 1, possibly due to the degradation of the electrolytesolution, and the current not associated with the formation of thetitanium oxide layer is observed. For this reason, it is more preferableto adjust the temperature of the electrolyte solution to 0° C. or less.Furthermore, when the temperature of the electrolyte solution isadjusted to −10° C. or less, the properties of the titanium oxide layerobtained by anodic oxidation no longer exhibit great differences even ifthe temperature of the electrolyte solution is changed. However, whenthe temperature of the electrolyte solution is adjusted to a lowtemperature of −30° C. or less, the amount of antifreezing agent such asethylene glycol added in order to prevent the freezing of electrolytesolution becomes too large. Hence, the level of the resistance ofelectrolyte solution increases when the temperature of the electrolytesolution is adjusted to −30° C. or less, thereby limiting the currentdensity at the time of conducting anodic oxidation to a low level.Therefore, it is preferable that the temperature of the electrolytesolution be not less than −30° C.

The anodic oxidation process in the present embodiment is conductedusing the above-mentioned electrolyte solution at the above-mentionedelectrolyte solution temperature, and carrying out constant-currentanodic oxidation using the substrate as an anode under the conditionsof, for example, a current density of 0.1 to 1,000 mA/cm², a voltage of2 to 400 V and a duration of 1 msec to 400 min, and then carrying outconstant-current anodic oxidation after having reached a specifiedvoltage. It should be noted that it is more preferable to conduct theanodic oxidation under the conditions consisted of a current density of0.1 to 100 mA/cm², a voltage of 5 to 90 V and a duration of 1 sec to 300min.

Note that there is a correlation between the thickness of the titaniumoxide layer obtained as a result of anodic oxidation and theabove-mentioned conditions for the anodic oxidation process, such as thematerial used for the substrate 1 and the voltage applied during theanodic oxidation. Therefore, by adjusting the above-mentioned conditionsfor the anodic oxidation, it is possible to appropriately adjust thethickness of the titanium oxide layer.

The substrate 1 on which the formation of the dielectric layer 2composed of an amorphous titanium oxide layer by anodic oxidation hasbeen completed is dried, following the removal of electrolyte solutionby washing. Thereafter, the masking tape is peeled off, and the counterelectrode 3 is formed on top of the dielectric layer 2. The counterelectrode 3 can be obtained by depositing a film made of metal such ascopper with a vacuum deposition method or the like. By forming thecounter electrode 3 in such a manner, the capacitor 10 shown in FIG. 1can be obtained.

Since the method for producing the capacitor 10 of the presentembodiment includes a step for forming an amorphous titanium oxidelayer, which is to become the dielectric layer 2, on top of thesubstrate 1 composed of titanium or an alloy containing titanium byanodizing the substrate 1 in an electrolyte solution containing hydrogenperoxide and having a temperature of 3° C. or less, the capacitor 10having a high withstand voltage and low leakage current can readily beobtained.

In addition, when the concentration of hydrogen peroxide in theelectrolyte solution is maintained from at least 0.1% by mass up to lessthan 50% by mass in the method for producing the capacitor 10 of thepresent embodiment, the effects due to the inclusion of hydrogenperoxide in the electrolyte solution can effectively be achieved.

Further, when the substrate 1 is composed of a titanium alloy in themethod for producing the capacitor 10 of the present embodiment, if thetitanium alloy is an alloy containing 70% by mass or more of titanium,the dielectric layer 2 composed of an amorphous titanium oxide layer canbe readily formed on the surface of the substrate 1.

Moreover, the capacitor 10 of the present embodiment is a capacitorproduced by the aforementioned method for producing the capacitor 10according to the present embodiment, and also includes the substrate 1serving as one electrode, the dielectric layer 2 formed on top thesubstrate 1, and the counter electrode formed on top of the dielectriclayer 2, wherein the dielectric layer 2 is composed of an amorphoustitanium oxide layer. Therefore, the capacitor 10 exhibits a highwithstand voltage and low leakage current.

Furthermore, by making the titanium oxide layer constituting thedielectric layer 2 so as to have a large relative dielectric constantwithin a range from 30 to 50, the capacitor 10 can be configured so asto have a high capacitance density.

In addition, according to the method for producing the capacitor 10 ofthe present embodiment, the capacitor 10 of high quality in which theproduct of capacitance density and dielectric breakdown voltage at ameasuring frequency of 1 kHz is 200 nF·V/cm² or more can be readilyobtained.

Further, according to the method for producing the capacitor 10 of thepresent embodiment, the capacitor 10 of high quality in which thedielectric loss tangent (tan δ) at a measuring frequency of 1 kHz is0.01 or less can be readily obtained.

Moreover, according to the method for producing the capacitor 10 of thepresent embodiment, the capacitor 10 in which the electrostaticcapacitance at a measuring frequency of 1 MHz is 80% or more of theelectrostatic capacitance at a measuring frequency of 100 Hz can bereadily obtained. Furthermore, since the capacitor 10 of the presentembodiment is a capacitor exhibiting a low degree of frequencydependency in which the electrostatic capacitance at a measuringfrequency of 1 MHz is 80% or more of the electrostatic capacitance at ameasuring frequency of 100 Hz, it can suitably be used for electronicdevices such as a wiring board and a high frequency module.

Additionally, according to the method for producing the capacitor 10 ofthe present embodiment, the dielectric layer 2 that is satisfactorilythin can be formed. Due to the above configuration, various effects canbe achieved, for example, the capacitor 10 can be made to have a highelectrostatic capacitance, the size of the capacitor 10 can be reduced,and the capacitor 10 can be readily formed within a wiring board or thelike.

OTHER EXAMPLES

Note that the present invention is not limited to the embodimentsdescribed above.

For example, although the capacitor 10 shown in FIG. 1 is configured sothat the dielectric layer 2 is composed of an amorphous titanium oxidelayer, the dielectric layer may be any dielectric layer as long as itincludes an amorphous titanium oxide layer, and may also be a laminatedbody in which an amorphous titanium oxide layer and an insulatingmaterial layer are laminated.

As such a laminated body, for example, it can be configured so that alayer composed of an insulating material, such as a composite oxidelayer that includes perovskite crystals in the form of barium titanateor the like, is further laminated on top of a titanium oxide layer.Examples of the method for forming such a composite oxide layer on topof a titanium oxide layer include a method in which an aqueous solutioncontaining at least one type of metal ion selected from the groupconsisting of Ca, Sr and Ba is reacted with the titanium oxide layerwithin a temperature range from 80° C. to the boiling point of theaqueous solution. By configuring the dielectric layer constituting acapacitor as such a laminated body, a capacitor having an even higherwithstand voltage with an even lower leakage current can be obtained.

FIG. 2 is a schematic cross sectional view showing another example of acapacitor according to the present invention. A capacitor 20 shown inFIG. 2 is a capacitor having a larger capacitance by laminating thecapacitor 10 shown in FIG. 1 in parallel.

Further, FIG. 3 is a schematic cross sectional view showing yet anotherexample of a capacitor according to the present invention. A capacitor30 shown in FIG. 3 includes, just the same as the capacitor 10 shown inFIG. 1, a substrate 1 a that also serves as one electrode, a dielectriclayer 2 a formed on top of the substrate 1 a and a counter electrode 3 a(the other electrode) formed on top of the dielectric layer 2 a. Asshown in FIG. 3, the capacitor 30 can be formed in series by making oneelectrode as a common electrode and splitting the other electrode.

(Field of Application)

The capacitor of the present invention can be suitably used in wiringboards and electronic devices, and is particularly suited as a componentfor portable electronic equipment represented by IC cards and mobilephones. Note that the process itself for implementing the capacitor in awiring board or electronic device can be carried out using ordinarymethods.

Since the capacitor of the present invention can be made thin, it can besuitably used as an electronic component, such as a capacitor within aboard, and enables the reduction of electronic components in size aswell as the reduction of electronic devices containing these electroniccomponents in both size and weight.

EXAMPLES

Although the present invention will be described below in more detailusing a series of Examples and Comparative Examples, the presentinvention is in no way limited to these Examples alone.

Example 1

A rectangle foil made of pure titanium having a thickness of 50 μm(manufactured by Thank Metal Co., Ltd.) was etched with hydrofluoricacid to a thickness of 40 μm for use as a substrate having a titaniumcontent shown in Table 1.

Next, a masking tape (ELEP Masking N380: manufactured by Nitto DenkoCorporation) was attached onto one side of the substrate with which apretreatment step had already been completed. Subsequently, an electrodeterminal for anodic oxidation was fixed to one side of the substrate.

Thereafter, an anodic oxidation treatment was carried out on a 10cm²-sized portion of the substrate ranging from the edge of the sidewhich was opposite to the fixed side of the substrate, in an electrolytesolution having a temperature (° C.) and composition indicated in Table1, at a current density (mA/cm²) indicated in Table 1, and with aconstant current until the anodic oxidation voltage (V) indicated inTable 1 was achieved. Furthermore, after the anodic oxidation voltage(V) indicated in Table 1 was achieved, an anodic oxidation treatment wascarried out at a constant voltage until the duration of 10 minutes hadpassed from the start of the anodic oxidation process, thereby forming atitanium oxide layer.

TABLE 1 Ti content Temperature Current Anodic in substrate ofelectrolyte density oxidation (%) Composition of electrolyte solutionsolution (° C.) (mA/cm²) voltage Ex. 1 99.5 Water:Ethyleneglycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:1 −10 1 60 Ex. 2 99.5Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:1 0 160 Ex. 3 99.5 Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide =72:24:3:1 3 1 60 Ex. 4 99.5 Water:Ethylene glycol:Phosphoricacid:Hydrogen peroxide = 72:24:3:1 −30 1 60 Comp. Ex. 1 99.5Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:1 5 160 Comp. Ex. 2 99.5 Water:Ethylene glycol:Phosphoric acid:Hydrogenperoxide = 72:24:3:1 10 1 60 Comp. Ex. 3 99.5 Water:Ethyleneglycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:1 25 1 60 Comp. Ex. 499.5 Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:160 1 60 Ex. 5 99.5 Water:Ethylene glycol:Phosphoric acid:Hydrogenperoxide = 72.8:24:3:0.2 −10 1 60 Ex. 6 99.5 Water:Ethyleneglycol:Phosphoric acid:Hydrogen peroxide = 72.9:24:3:0.1 −10 1 60 Ex. 799.5 Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide =53:24:3:20 −10 1 60 Ex. 8 99.5 Water:Ethylene glycol:Phosphoricacid:Hydrogen peroxide = 33:24:3:40 −10 1 60 Ex. 19 99.5 Water:Ethyleneglycol:Phosphoric acid:Hydrogen peroxide = 72.95:24:3:0.05 −10 1 60 Ex.20 99.5 Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide =40:7:3:50 −10 1 60 Ex. 9 76 Water:Ethylene glycol:Phosphoricacid:Hydrogen peroxide = 72:24:3:1 −10 1 60 Ex. 10 99.5 Water:Ethyleneglycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:1 −10 1 5 Ex. 11 99.5Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:1 −101 2 Ex. 12 99.5 Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide= 72:24:3:1 −10 1 1 Ex. 13 99.5 Water:Ethylene glycol:Phosphoricacid:Hydrogen peroxide = 72:24:3:1 −10 1 90 Ex. 14 99.5 Water:Ethyleneglycol:Ammonium adipate:Hydrogen peroxide = 72:24:3:1 −10 1 60 Ex. 1599.5 Water:Ethylene glycol:Sulfuric acid:Hydrogen peroxide = 72:24:3:1−10 1 60 Ex. 16 99.5 Water:Ethylene glycol:Phosphoric acid:Hydrogenperoxide = 72:24:3:1 −10 0.1 60 Ex. 17 99.5 Water:Ethyleneglycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:1 −10 100 60 Ex. 1899.5 Water:Ethylene glycol:Phosphoric acid:Hydrogen peroxide = 72:24:3:1−10 300 60 Comp. Ex. 5 0

The descriptions on the compositional ratio regarding the composition ofelectrolyte solution shown in Table 1 indicate the mass ratio of eachcompound.

Next, the substrate on which a titanium oxide layer was formed wasrinsed with water and then dried.

The surface of the obtained titanium oxide layer was then observed witha scanning electron microscope (SEM). The results are shown in FIG. 4.FIG. 4 is an SEM micrograph of the surface of a titanium oxide layer. Asshown in FIG. 4, the surface of the titanium oxide layer obtained inExample 1 was smooth.

(Film Thickness of Titanium Oxide Layer)

In addition, the substrate on which a titanium oxide layer was formedwas cut out with a focused ion beam (FIB) device and the resultingcross-sectional structure of the substrate was observed with atransmission electron microscope (TEM), thereby measuring the thicknessof the titanium oxide layer. The results are shown in FIG. 5. FIG. 5 isa TEM micrograph of a cross section of a titanium oxide layer.

The film thickness of the titanium oxide layer (anodized layer) obtainedin Example 1 shown in FIG. 5 is shown in Table 2.

Further, as shown in FIG. 5, it was confirmed that the titanium oxidelayer obtained in Example 1 was amorphous with no visible crystallizeddomains.

TABLE 2 Film Capacitance Relative Dielectric Direct current Dielectric(C (1 MHz)/ thickness Refractive density dielectric loss resistancebreakdown A × B C (100 Hz) (nm) index (nF/cm²) A constant tangent (Ω)voltage (nF · V/cm²) (%) Ex. 1 140 2.20 206 32.6 0.002 1.4 × 10⁹ 4.0 82498 Ex. 2 140 2.28 216 34.2 0.003 1.2 × 10⁹ 3.6 778 97 Ex. 3 140 2.34 21734.3 0.005 1.1 × 10⁹ 3.4 738 95 Ex. 4 140 2.07 204 32.3 0.002 1.5 × 10⁹4.1 836 98 Comp. Ex. 1 140 2.37 220 34.8 0.006 8.0 × 10⁸ 3.2 704 91Comp. Ex. 2 140 2.46 228 36.1 0.011 4.2 × 10⁸ 2.9 661 90 Comp. Ex. 3 1402.52 240 38.0 0.045 8.6 × 10⁷ 2.0 480 88 Comp. Ex. 4 140 2.55 357 56.50.088 2.2 × 10⁶ 0.9 321 69 Ex. 5 140 2.21 204 32.3 0.002 1.4 × 10⁹ 4.1836 98 Ex. 6 140 2.20 207 32.8 0.003 9.0 × 10⁸ 4.0 828 97 Ex. 7 140 2.22209 33.1 0.003 1.1 × 10⁹ 4.1 857 97 Ex. 8 140 2.20 211 33.4 0.003 1.1 ×10⁹ 3.8 802 96 Ex. 19 140 2.21 226 35.8 0.012 5.0 × 10⁸ 2.2 497 90 Ex.20 140 2.17 215 34.0 0.020 2.4 × 10⁸ 1.4 301 87 Ex. 9 140 2.10 316 50.00.009 4.3 × 10⁸ 1.3 411 85 Ex. 10 12 2.02 2,400 32.6 0.004 1.2 × 10⁸ 0.3720 95 Ex. 11 4.4 1.94 6,710 33.4 0.006 2.4 × 10⁷ 0.2 1,340 95 Ex. 12 21.91 16,800 38.0 0.009 9.8 × 10⁶ 0.1 1,680 92 Ex. 13 210 2.20 132 31.30.002 2.3 × 10⁹ 6.2 818 98 Ex. 14 140 2.15 211 33.4 0.008 7.9 × 10⁸ 3.0633 88 Ex. 15 140 2.24 203 32.1 0.005 1.1 × 10⁹ 3.8 771 97 Ex. 16 1402.11 200 31.7 0.003 1.3 × 10⁹ 4.4 880 98 Ex. 17 140 2.26 213 33.7 0.0058.8 × 10⁸ 3.5 746 94 Ex. 18 140 2.30 216 34.2 0.007 6.3 × 10⁸ 3.1 670 91Comp. Ex. 5 140 2.56 518 82.0 0.125 4.7 × 10⁵ 0.3 155 66

In addition, the elements which constituted the titanium oxide layerformed on top of the substrate were examined using an energy dispersiveX-ray spectrometer (EDS) attached to the TEM. The results are shown inFIG. 6. FIG. 6 is a graph showing the result of energy dispersive X-rayspectroscopy (EDS) of a titanium oxide layer. In FIG. 6, the horizontalaxis indicates energy while the vertical axis indicates intensity(counts). From the results shown in FIG. 6, it was possible to verifythat the titanium oxide layer obtained in Example 1 contained titaniumand oxygen as major components, and a small amount of phosphorus wasincorporated within the titanium oxide layer.

(Refractive Index)

In addition, the refractive index of the titanium oxide layer formed ontop of the substrate at a wavelength of 632.8 nm was measured using aDHA-XA type ellipsometer (product name, manufactured by Mizojiri OpticalCo., Ltd.). As a result, as shown in Table 2, the refractive index ofthe titanium oxide layer was 2.20.

(Density)

Further, the density of the titanium oxide layer obtained in Example 1was calculated from the Lorentz-Lorentz equation using 3.9 g/cm³ and2.56, which were the density and refractive index of anatase(crystalline titanium oxide) generally obtained when anodizing titaniumat a voltage of 100 V or less, respectively. As a result, the density ofthe titanium oxide layer obtained in Example 1 was equivalent to 3.4g/cm³.

Next, the masking tape was peeled off from the substrate, and a samplewas cut out to a size of 30 mm×30 mm. A mask having an opening was thenformed in the surface of the sample where the titanium oxide layer wasformed. Thereafter, copper was laminated inside the opening to athickness of 400 nm by an electron beam deposition method to form acounter electrode having a size of 10 mm×10 mm, thereby obtaining acapacitor in which the counter electrode was serving as a cathode whilethe substrate was serving as an anode. The thickness of the resultingcapacitor as a whole of Example 1 obtained in such a manner was 41 μm.

(Capacitance Density)

The electrostatic capacitance of the capacitor obtained in Example 1 wasmeasured using the LF Impedance Analyzer (Model 4192A, AgilentTechnologies Inc.) under the conditions of a measuring frequency of 1kHz and an amplitude of 1 V. As a result, a large value of 206 nF wasobtained for the electrostatic capacitance of the capacitor ofExample 1. Here, since the electrode area is 1 cm², the capacitancedensity of the capacitor obtained in Example 1 becomes 206 nF/cm² asindicated in Table 2.

(Relative Dielectric Constant)

In addition, the relative dielectric constant of the titanium oxidelayer constituting the capacitor obtained in Example 1 was calculatedfrom the electrostatic capacitance and the thickness of the titaniumoxide layer.

(Dielectric Loss Tangent (tan δ))

Further, the dielectric loss tangent (tan δ) of the capacitor obtainedin Example 1 at a measuring frequency of 1 kHz was determined.

(Direct Current Resistance)

Moreover, the capacitor obtained in Example 1 was connected in serieswith a 1 MΩ resistor, and a direct current voltage of 1 V was thenapplied to the capacitor and the 1 MΩ resistor and the voltage appliedto both ends of the 1 MΩ resistor was measured 30 seconds after applyingthe voltage. The direct current resistance of the capacitor wascalculated from the voltage value obtained above.

(Dielectric Breakdown Voltage)

Furthermore, the dielectric breakdown voltage (withstand voltage) of thecapacitor obtained in Example 1 was determined by the method shownbelow. First, a direct current applied voltage was increased in 0.1 Vincrements starting at 1 V using the same measuring circuit as that usedduring the measurement of direct current resistance described above.After applying each voltage for 30 seconds, the applied voltage wasreturned to 1 V and the direct current resistance was measured 30seconds later. The applied voltage when the direct current resistance 30seconds later had reduced to less than 1 MΩ was taken to be thedielectric breakdown voltage.

(C (1 MHz)/C (100 Hz))

The capacitor obtained in Example 1 was laminated between two flexiblecopper-clad laminates having a size of 100 mm×100 mm. The electrostaticcapacitance was measured using the LF Impedance Analyzer (Model 4192A,Agilent Technologies Inc.) at room temperature at a measuring frequencyof 100 Hz, 1 kHz and 1 MHz. As a result, the electrostatic capacitanceof the capacitor obtained in Example 1 was 208 nF at a measuringfrequency of 100 Hz, 207 nF at 1 kHz, and 204 nF at 1 MHz.

In addition, by using the capacitor obtained in Example 1 laminatedbetween two flexible copper-clad laminates having a size of 100 mm×100mm, the electrostatic capacitance was measured while continuouslychanging the temperature from −55° C. to +125° C. at a measuringfrequency of 1 kHz using the same LF Impedance Analyzer as describedabove. The electrostatic capacitance within this temperature range at 1kHz was a minimum of 204 nF and a maximum of 209 nF. From the result, itbecame clear that the dependency of electrostatic capacitance ontemperature was extremely low.

Further, by using the capacitor obtained in Example 1 laminated betweentwo flexible copper-clad laminates having a size of 100 mm×100 mm, theelectrostatic capacitance was measured, while applying a direct currentbias voltage of 1.5 V via the flexible copper-clad laminates so that thesubstrate of the capacitor was at a positive electric potential, at ameasuring frequency of 1 kHz using the same LF Impedance Analyzer asdescribed above.

As a result, the electrostatic capacitance of the capacitor obtained inExample 1 was 204 nF. From the result, it became clear that thedependency of electrostatic capacitance on bias voltage was extremelylow.

The relative dielectric constant, dielectric loss tangent (tan δ),direct current resistance, dielectric breakdown voltage, the product ofcapacitance density and dielectric breakdown voltage at a measuringfrequency of 1 kHz, and the ratio between the electrostatic capacitanceat a measuring frequency of 1 MHz and the electrostatic capacitance at ameasuring frequency of 100 Hz (i.e., C (1 MHz)/C (100 Hz)), with respectto the titanium oxide layer of the capacitor of Example 1 obtained inthe above-mentioned manner are shown in Table 2.

Examples 2 to 4, Comparative Examples 1 to 4

A titanium oxide layer was formed in the same manner as in Example 1with the exception that the temperature of the electrolyte solution waschanged to a temperature indicated in Table 1. The film thickness andrefractive index of the titanium oxide layer were determined in the samemanner as in Example 1. The results are shown in Table 2. In addition, acapacitor was formed in the same manner as in Example 1 using asubstrate on which a titanium oxide layer had been formed, and thecapacitor characteristics indicated in Table 2 were examined in the samemanner as in Example 1. The results are shown in Table 2.

Examples 5 to 8, 19 and 20

A titanium oxide layer was formed in the same manner as in Example 1with the exception that the composition of the electrolyte solution waschanged to a composition indicated in Table 1. The film thickness andrefractive index of the titanium oxide layer were determined in the samemanner as in Example 1. The results are shown in Table 2. In addition, acapacitor was formed in the same manner as in Example 1 using asubstrate on which a titanium oxide layer had been formed, and thecapacitor characteristics indicated in Table 2 were examined in the samemanner as in Example 1. The results are shown in Table 2.

Example 9

A titanium oxide layer was formed in the same manner as in Example 1with the exception that a foil composed of the AMS 4914 material (havinga composition of titanium (76%), V (15%), Cr (3%), Sn (3%) and Al (3%))with a thickness of 50 μm was used as a substrate. The film thicknessand refractive index of the titanium oxide layer were determined in thesame manner as in Example 1. The results are shown in Table 2. Inaddition, a capacitor was formed in the same manner as in Example 1using a substrate on which a titanium oxide layer had been formed, andthe capacitor characteristics indicated in Table 2 were examined in thesame manner as in Example 1. The results are shown in Table 2.

Examples 10 to 13

A titanium oxide layer was formed in the same manner as in Example 1with the exception that the anodic oxidation voltage was changed to avoltage indicated in Table 1. The film thickness and refractive index ofthe titanium oxide layer were determined in the same manner as inExample 1. The results are shown in Table 2. In addition, a capacitorwas formed in the same manner as in Example 1 using a substrate on whicha titanium oxide layer had been formed, and the capacitorcharacteristics indicated in Table 2 were examined in the same manner asin Example 1. The results are shown in Table 2.

Example 14

A titanium oxide layer was formed in the same manner as in Example 1with the exception that, instead of phosphoric acid contained in theelectrolyte solution, ammonium adipate of an equal weight was used. Thefilm thickness and refractive index of the titanium oxide layer weredetermined in the same manner as in Example 1. The results are shown inTable 2. In addition, a capacitor was formed in the same manner as inExample 1 using a substrate on which a titanium oxide layer had beenformed, and the capacitor characteristics indicated in Table 2 wereexamined in the same manner as in Example 1. The results are shown inTable 2.

Example 15

A titanium oxide layer was formed in the same manner as in Example 1with the exception that, instead of phosphoric acid contained in theelectrolyte solution, sulfuric acid of an equal weight was used. Thefilm thickness and refractive index of the titanium oxide layer weredetermined in the same manner as in Example 1. The results are shown inTable 2. In addition, a capacitor was formed in the same manner as inExample 1 using a substrate on which a titanium oxide layer had beenformed, and the capacitor characteristics indicated in Table 2 wereexamined in the same manner as in Example 1. The results are shown inTable 2.

Examples 16 to 18

A titanium oxide layer was formed in the same manner as in Example 1with the exception that the current density was changed to a currentdensity indicated in Table 1. It should be noted that because a voltageof 60 V was not achieved with the current density of Example 16 withinthe same duration as that of Example 1, the duration of anodic oxidationprocess in Example 16 was adjusted to 60 minutes. Apart from the abovemodification, a titanium oxide layer was formed and the film thicknessand refractive index of the titanium oxide layer were determined in thesame manner as in Example 1. The results are shown in Table 2. Inaddition, a capacitor was formed in the same manner as in Example 1using a substrate on which a titanium oxide layer had been formed, andthe capacitor characteristics indicated in Table 2 were examined in thesame manner as in Example 1. The results are shown in Table 2.

Comparative Example 5

A platinum foil having a size of 15 mm×15 mm and having a thickness of50 μm was prepared as a substrate, and the substrate and a sputteringtarget were placed inside the chamber of the sputtering apparatus sothat the distance between the substrate and the sputtering target was 50mm. The Model SPF-332H (manufactured by Nichiden Anelva Corporation) wasused as the sputtering apparatus. Further, titanium dioxide having arutile crystal structure with a diameter of 76 mm and a thickness of 5mm was used as the sputtering target.

The pressure inside the chamber was then reduced without heating thesubstrate until a pressure of 2×10⁻⁴ Pa was achieved. Subsequently,after introducing oxygen until the pressure inside the chamber reached apressure of 1×10⁻² Pa, argon was introduced until the pressure insidethe chamber reached a pressure of 2×10⁻¹ Pa. Thereafter, a radiofrequency (RF) power was set to 200 W, and a titanium dioxide filmhaving a thickness indicated in Table 2 was formed on top of thesubstrate by sputtering the sputtering target.

The substrate on which a titanium dioxide film had been formed was thentaken out of the chamber, and the refractive index of the titaniumdioxide film was measured in the same manner as in Example 1. Theresults are shown in Table 2.

In addition, a capacitor was formed in the same manner as in Example 1using a substrate on which a titanium oxide layer had been formed withthe exception that the thickness of copper deposition was changed to 200nm, and the capacitor characteristics indicated in Table 2 were examinedin the same manner as in Example 1. The results are shown in Table 2.

From the results shown in Table 2, it was possible to confirm that therefractive index of the titanium oxide layer in Examples 1 to 20 at awavelength of 632.8 nm was within a range from 1.90 to 2.35, and thatthe titanium oxide constituting the titanium oxide layer was amorphous.In addition, in Examples 1 to 20, the relative dielectric constant ofthe titanium oxide layer was within a range from 30 to 50, and theproduct of capacitance density and dielectric breakdown voltage at ameasuring frequency of 1 kHz was 200 nF·V/cm² or more.

Moreover, the ratio (C (1 MHz)/C (100 Hz)) was 80% or more in Examples 1to 20, showing a low level of frequency dependency.

Furthermore, in Examples 1 to 18 in which the concentration of hydrogenperoxide in the electrolyte solution was within a range from 0.1 to 40%by mass, a capacitor 10 exhibiting a dielectric loss tangent (tan δ), ata measuring frequency of 1 kHz, of 0.01 or less was obtained.

On the other hand, in Comparative Examples 1 to 5, since the temperatureof the electrolyte solution was higher than 3° C., the refractive indexof the titanium oxide layer at a wavelength of 632.8 nm exceeded 2.35.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a method for producing acapacitor, a capacitor, and a wiring board, electronic device and ICcard which include the capacitor, and can be applied particularly to amethod for producing a capacitor which is capable of producing acapacitor having a high withstand voltage and low leakage current.

1.-8. (canceled)
 9. A capacitor produced using a method for producing acapacitor having a substrate serving as one electrode, a dielectriclayer formed on top of the substrate, and the other electrode formed ontop of the dielectric layer, the method comprising: a step for formingan amorphous titanium oxide layer which is to become the dielectriclayer on top of the substrate by anodizing the substrate, which iscomposed of titanium or titanium alloy, in an electrolyte solutioncontaining hydrogen peroxide and having a temperature of 3° C. or less;and a step for forming the other electrode on top of the dielectriclayer.
 10. A capacitor having a substrate serving as one electrode, adielectric layer formed on top of the substrate, and the other electrodeformed on top of the dielectric layer, the capacitor comprising thesubstrate composed of titanium or titanium alloy; and the dielectriclayer including an amorphous titanium oxide layer.
 11. The capacitoraccording to claim 10, wherein the dielectric layer is a laminated bodyincluding the amorphous titanium oxide layer and an insulating materiallayer.
 12. The capacitor according to claim 9, wherein a refractiveindex of the amorphous titanium oxide layer at a wavelength of 632.8 nmis within a range from 1.90 to 2.35.
 13. The capacitor according toclaim 9, wherein a relative dielectric constant of the amorphoustitanium oxide layer is within a range from 30 to
 50. 14. The capacitoraccording to claim 9, wherein the product of capacitance density anddielectric breakdown voltage at a measuring frequency of 1 kHz is 200nF·V/cm² or more.
 15. The capacitor according to claim 9, wherein adielectric loss tangent at a measuring frequency of 1 kHz is 0.01 orless.
 16. The capacitor according to claim 9, wherein an electrostaticcapacitance at a measuring frequency of 1 MHz is 80% or more of anelectrostatic capacitance at a measuring frequency of 100 Hz.
 17. Awiring board comprising the capacitor described in claim
 9. 18. Anelectronic device comprising the capacitor described in claim
 9. 19. AnIC card comprising the capacitor described in claim
 9. 20. The capacitoraccording to claim 10, wherein a refractive index of the amorphoustitanium oxide layer at a wavelength of 632.8 nm is within a range from1.90 to 2.35.
 21. The capacitor according to claim 10, wherein arelative dielectric constant of the amorphous titanium oxide layer iswithin a range from 30 to
 50. 22. The capacitor according to claim 10,wherein the product of capacitance density and dielectric breakdownvoltage at a measuring frequency of 1 kHz is 200 nF·V/cm² or more. 23.The capacitor according to claim 10, wherein a dielectric loss tangentat a measuring frequency of 1 kHz is 0.01 or less.
 24. The capacitoraccording to claim 10, wherein an electrostatic capacitance at ameasuring frequency of 1 MHz is 80% or more of an electrostaticcapacitance at a measuring frequency of 100 Hz.
 25. A wiring boardcomprising the capacitor described in claim
 10. 26. An electronic devicecomprising the capacitor described in claim
 10. 27. An IC cardcomprising the capacitor described in claim 10.